OK, After slaughtering countless reavers I can confirm this item does drop but it comes from a very specific reaver.
The ONLY reaver that can drop this (as far as I can tell) is the one that sits on the platform up the elevator, in front of the 3 spawn. The same Black Reaver that shares a spawn with Tzirathk. It only has the chance to drop when that specific reaver dies, and will never drop from any additional reavers spawned.
Good luck.